My reasons for diversifying income are simple: I want to be able to quit my day job eventually. But your reasons may be different, maybe your job isn’t that secure or your co-workers are starting to feel the pressure. It really doesn’t matter why you diversify your sources of income, what matters is that you do it. Making money won’t happen overnight with second sources of income so if you wait until it’s too late you’ll be screwed.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.